The present invention relates to a head device for use in a magnetic disk storage apparatus and particularly, a read head provided with a spin-valve magnetoresistive (MR) element.
Recently, a hard disk drive (HDD) includes a head for reading and writing data on a disk(s) which comprises a write head incorporating an inductive film head and a read head incorporating an MR (magnetoresistive) head.
The MR head includes an MR element having a magnetoresistive effect to a magnetic field (the signal magnetic field derived from a disk) and thus has several times characteristic higher in the reproducing output than an inductive film head. Recently, an improved type of the MR head has been focused including a spin-valve MR element is provided with spin-valve layers. Such an improved MR head is known as a GMR (giant magnetoresistive) head.
Referring to FIG. 16, a spin-valve layer structure 160 is made of a four-layer construction. More specifically, a non-magnetic layer (a conductive layer) 163 is sandwiched between two magnetic layers 161 and 162 and an antiferromagnetic layer 164 also called an exchange layer is provided on the magnetic layer 161. The antiferromagnetic layer 164 has a function for determining the magnetization of the magnetic layer 161 in one direction. The magnetic layer 161 is thus called a pinning layer. The magnetic layer 162 is a soft magnetic layer of which magnetizing direction is determined by an external signal magnetic field from the outside (namely, a disk) and also called a free layer (or a magnetic field detecting layer).
The principle of action in the spin-valve layer 160 is explained referring to FIGS. 17A and 17B.
As shown in FIG. 17A, the spin-valve layer 160 produces a parallel magnetizing alignment where the magnetizing direction 171 of the free layer 162 aligns with the magnetizing direction 172 of the pinning layer 161. In the alignment, electrons 173 can freely travel between the free layer 162 and the non-magnetic layer 163 thus decreasing the overall resistance of the spin-valve layer 160. Also, there is produced an antiparallel magnetizing alignment where the magnetizing direction 171 of the free layer 162 is opposite to the magnetizing direction 172 of the pinning layer 161, as shown in FIG. 17B. In this case, the electrons 173 are dispersed between the free layer 162 and the non-magnetic layer 163, hence sharply increasing the overall resistance of the spin-valve layer 160. Accordingly, a rate of resistance change (a resistance change rate) is increased to several times higher than that of a conventional (form anisotropic) MR head, which is known as a GMR effect. The spin-valve layer 160 having such a GMR effect is now favorable for use as a primary element in a next-generation MR head (or a GMR head) for recording and reproducing a high density record.
FIG. 18 is a schematic view of a normal arrangement of a read/write head 2 using the spin-valve layer 160. The read/write head comprises mainly of a read head 180 and a write head 181 mounted separately. The read head 180 is a GMR head having the spin-valve layer 160. The spin-valve layer 160 is sandwiched between an upper shield member 182 and a lower shield member 183 for protection from the magnetic fields generated by adjacent data. The upper shield member 182 also functions as a lower electrode for the write head 181 as explained later. The distance between the two shield members 182 and 183 is a read gap RG of the read head 180. In addition, a pair of hard magnets 184a and 184b are provided with ferromagnetic layers (or semi-ferromagnetic layers) for determining the magnetization of the free layer 162 in one direction, on both sides of the spin-valve layer 160. The spin-valve layer 160 is connected by the hard magnets 184a and 184b to leads 185a and 185b respectively.
The write head 181 includes a coil 186 made of a spiral pattern form of a conductive material. The pattern form of the coil 186 is encapsulated by an insulating material 187 such as alumina (Al.sub.2 O.sub.3). The coil 186 generates a magnetic field across a write gap WG, when supplied with a (write) current for data write action. A ring by the coil 186 extends through a tubular space defined by an upper electrode 188 and the lower electrode (or the upper shield member of the read head 180) 182. The foregoing element structure is fabricated by a head manufacturing process in which a thin film forming method is applied on a substrate 189 of the head forming a slider.
For using the read/write head as a magnetic head device in an HDD, it is desired that the resistance change in the spin-valve layer 160 of the read head is proportional to a leak magnetic flux (of the signal magnetic field) from a disk or a magnetic storage medium in the HDD (see the H-R characteristic curve shown in FIG. 19). As shown in FIG. 19, the read head having the spin-valve layer 160 is designed so that the magnetizing direction 201 of the pinning layer 161 is vertical to (a signal magnetic field 190 of) the disk located in its opposite position and the magnetizing direction 202 (at the initial state) of the free layer 162 is parallel to the disk, producing an orthogonal magnetizing alignment state (referred to as HEAD-A). More particularly, the magnetizing directions 201 and 202 of the two layers 161 and 162 are at a right angle to each other, hence allowing the magnetizing direction 202 of the free layer 162 to be (magnetically) shifted in proportion to a magnitude of the leak magnetic field 190 from the disk and thus the spin-valve layer 160 to be varied in the resistance value corresponding to an angle of the shift (for example, a degree defined between the direction 202 and a direction 200).
The antiferromagnetic layer 164 which is a primary member in the spin-valve layer 160 is omitted in FIG. 19. FIG. 19 also shows H-R characteristic in a parallel magnetizing state (HEAD-B) and an antiparallel magnetizing state (HEAD-C) for comparison with the favorable state HEAD-A.
As described, it is essential in the read head (a GMR head) of the HDD that the pinning layer 161 of the spin-valve layer 160 has its magnetizing direction determined vertically to the disk. Meanwhile, the magnetizing direction of the pinning layer 161 is controlled and maintained by an exchange magnetic field on the antiferromagnetic layer 164. The magnitude (or intensity) of the exchange magnetic field is decreased with temperature and becomes zero when the temperature is a predetermined degree which is known as a blocking temperature. The blocking temperature of the antiferromagnetic layer made generally of an (Fe--Mn) alloy of iron and manganese is 150.degree. C. When the ambient temperature about the spin-valve layer 160.degree. C. exceeds 150.degree. C., the magnetizing direction of the pinning layer 161 is shifted.
Also, it is proved in a process of manufacturing a GMR head, a process of assembling the GMR head in an HDD, or an action of the HDD having the GMR head that the magnetizing direction of the pinning layer 161 in the spin-valve layer 160 is likely to shift due to any combination of the following four factors (1) to (4).
(1) Ambient temperature
As described, when the GMR head is operated at a temperature exceeding the blocking temperature or the ambient temperature about the spin-valve layer 160 after determining magnetizing direction, exceeds the blocking temperature, the magnetizing direction of the pinning layer 161 may be shifted. With the GMR head assembled in an HDD, the temperature in the HDD is increased more or less to 20.degree. C. due to heat generated by electric circuits and a motor. Generally, the temperature allowing a proper action of the HDD is about 60.degree. C. The MR head (including GMR head) is fed with an operating current which is also called a sense current. Since the sense current generates heat, the temperature in the HDD is increased up to 40.degree. C. Depending on extra conditions, the ambient temperature about the spin-valve layer 160 in the HDD may rise close to 120.degree. C.
(2) Magnetic field from external source
Even if the ambient temperature is below the blocking temperature with the GMR head under its operable condition, development of an external magnetic field about the spin-valve layer 160 which is higher in the magnitude than the exchange magnetic field may shift the magnetizing direction of the pinning layer 161.
(3) ESD (electrostatic discharge)
When an instantaneous overcurrent is produced by an effect of ESD, it may break down the GMR element (the spin-valve layer 160). The overcurrent and decreases the amplitude of the exchange magnetic field, even if its rate is relatively small effects. The overcurrent generates heat and may introduce a magnetic field which is oriented opposite to the magnetizing direction of the pinning layer 161, due to the direction of the current. In other words, the effect of ESD stimulates the above two factors (1) and (2) at once. A specific degree of ESD may cause shifting of the magnetizing direction of the pinning layer 161. (4) Combination of the factors (1) to (3).
When the magnetizing direction of the pinning layer 161 is shifted from its correct orientation by an adverse effect of any combination of the three factors (1) to (3), the magnetic rotation on the free layer 162 triggered by the leak magnetic field from the disk may exhibit a different action from that based on a change in the resistance. This permits the read head to malfunction.
For compensation, it may be possible to provide an improved antiferromagnetic layer having a proper blocking temperature response and control the temperature in a process of manufacturing an MR head to a desired degree. However, either the effect of external magnetic fields depicted in the paragraph (2) or the effect of ESD depicted in the paragraph (3) is hardly controlled and will thus lead to the malfunction of the read head. It is also quite difficult to identify all the drawbacks and discriminate defective heads during the step of inspection before assembling the read/write head to the HDD. The exchange magnetic field declines due to the effect of ESD. Also, the read heads may be declined in the performance after operating a predetermined time, by temperature rise and time during HDD operating. If any of such troubles occurs after the GMR head is assembled as the read head in the HDD, it will prevent the user to read data from the disk in the HDD.